Do the "Mega Force", "Increased Units" and "Drain Units" options affect all nations? I just attacked a country, and when activated the cheats, the enemy unit size were also changed. Have I done something wrong?

Yes and no on the unit size, if you are doing it close to a neighbouring country they prolly will get the same amount that you add but try to add it as far from you if you can from any border and it should work

Every time I try to blow up someone, they won't let me use nukes, and strike, and get ****ed off, and then boot my ass out the door..... Even with the really red places, I.E China and Japan, or Russia and Georgia. Any tips ?